1. Field of the Invention
The present invention relates to a solar module having a solar concentrator and especially to a solar concentrator having an improved converging.
2. Description of the Related Art
Heretofore, it has been well known that there is technique to reduce an area to be used for expensive solar cell by converging sunlight through a lens for the purpose of reducing the costs of a power generation system which uses solar batteries. Also, a technique to track the sun for the purpose of improving the generation efficiency of a solar module is well known. However, such a tracking system leads to high driving costs of the solar module and requires a lot of man-hours for maintenance. Therefore, fixed type solar concentrators have been conceived.
Japanese Patent Laid-Open Publication No. Hei 2-203160 discloses such a fixed type solar concentrator. This conventional fixed type solar concentrator has a so called tapering constitution as shown in FIG. 16. More specifically, an internal mirror is constructed in such a manner that the diameter of the mirror on the solar cell side is shorter than the diameter on the other side.
However, in the conventional solar concentrator described above, a part of the sunlight 10 incident on the taperingly constructed internal mirror 100 performs repetitive reflection on the internal mirror 100 and finally goes out in the direction of incidence, namely, the direction back toward the sky. Therefore, there is a problem that in a solar concentrator using the taperingly constructed internal mirror 100, the converging magnification cannot be improved.
FIG. 17 exemplifies a device which converges the sunlight 10 on the reception surface of a solar cell 12 by using the taperingly constructed internal mirror 100 as shown in FIG. 16. In FIG. 17, it is defined that an incident angle is an angle .theta. between a direction of the sunlight 10 and a normal line 14 which is made perpendicular to the reception surface of the solar cell 12.
FIG. 18 shows a relation between the incident angle mentioned above and outgoing intensity as the proportion of partial sunlight incident on the reception surface of the solar cell 12 to all the sunlight incident on the solar concentrator in the solar module shown in FIG. 17. The axis of ordinates shows the outgoing intensity and the axis of abscissas shows the incident angle, respectively. The solar concentrator shown in FIG. 17 is 60 mm in height. The diameter of an incident port is 40 mm and the diameter of an outgoing port is 8 mm. The converging ratio of the solar concentrator is 2.5 to 1.
As shown in FIG. 18, with the increase of incident angle, outgoing intensity falls, and when the incident angle exceeds an angle of 10 degrees, the outgoing intensity drops to 25 percent or less. As described above, this is because with the increase of the incident angle, an amount of sunlight which goes out in the direction of the incidence increases due to the reflection on the taperingly constructed internal mirror 100.
As described above, the converging magnification cannot be improved in such a tapering type solar concentrator due to the existence of the sunlight which goes out in the direction of the incidence. On the other hand, in a solar concentrator in which a section of the internal mirror 102 is arranged with parallel sides as shown in FIG. 19 or a solar concentrator in which a section of the internal mirror 102 is arranged in such a manner that the diameter of the mirror widens toward the solar cell as shown in FIG. 20, the incident sunlight will not go out in the direction of the incidence during the repetition of reflection. However, since an incident port and an outgoing port are of the same size or the former is smaller than the latter, it is impossible to improve the converging magnification.
Also, Japanese Patent Laid-open Publication No. Sho 54-18762 discloses a solar concentrator composed of a prism which has a triangular section and performs convergence by utilizing reflection and total internal reflection. In FIG. 21, constitution of the prism 16 disclosed in the aforementioned reference is exemplified. Also, in an example of FIG. 22, a pair of prisms 16 the mate of which is shown in FIG. 21 are arranged facing each other in a V shape. In the examples of constitution shown in FIGS. 21 and 22, a surface arranged opposite to an incident surface 20 is a mirror surface 18. The sunlight 10 which comes in through the incident surface 20 is reflected on the mirror surface 18. The sunlight 10 then returns to the incident surface 20 again. It performs total internal reflection there and enters the solar cell 12.
However, in the constitution shown in FIG. 21, in order to satisfy the conditions of total internal reflection on the incident surface 20, an apex angle .alpha. of the prism 16 has to be larger than a prescribed angle. Therefore, there has been such a problem that a ratio of the surface area of the solar cell 12 to the surface area of the incident surface 20 is automatically determined, thereby making it impossible to obtain a converging magnification which exceeds a prescribed value.
Also in the constitution shown in FIG. 22, for the purpose of arranging two prisms 16 face to face, the corresponding solar cells 12 are arranged on the bottom face of the prism 16 not in a straight line, but in such a way that they define a prescribed angle with respect to each other. Consequently, in the solar concentrator according to this embodiment, mounting in an actual device is practically difficult.
The constitution as shown in FIG. 23 can also serve the purpose of arranging two prisms 16 face to face. More specifically, in order to have a flat bottom face on the apex side of a V-shaped substance, there is provided an extension 30 which is extended further from the apex of the V-shaped substance. Due to such a constitution, the solar cells 12 can be arranged in a straight line. In this case, however, as the extension 30 is in a tapering form as shown in FIG. 16, total internal reflection may be repeatedly performed at that part and the sunlight 10 may go out in the direction of incidence. Therefore, in order to form the extension 30 which is further extended from the apex of the V-shaped substance, it is necessary to optimize conditions of the constitution.